[Suggestion] Add prompt and tool snapshot diffs #41

Is your feature request related to a problem? Please describe.
The plugin stores the latest system prompt and tool definitions, so users cannot see what changed when context size or cost increases.

Describe the solution you'd like
Retain bounded snapshot history and show added, removed, and changed system fragments, skills, references, MCP instructions, and tool definitions between turns or sessions.

Additional context
Execution order: 9, after #40. Keep raw content local, bounded, and compatible with the existing snapshot retention model.